1. Field of the Invention
The present invention relates to a fuser construction and, more particularly, to a method and apparatus for controlling an axial temperature distribution in a fuser.
2. Related Prior Art
In an electrophotographic image forming apparatus, such as a printer or copier, a latent image is formed on a light sensitive drum and developed with toner. The toner image is then transferred onto a medium, such as a sheet of paper, and is subsequently passed through a fuser where heat is applied to melt the toner and fuse it to the medium. The fuser includes a fuser roller cooperating with a backup member to form a nip through which the toned media passes. The fuser roller may be provided with an internal heater, such as a halogen lamp, and the temperature of the fuser roller is monitored by a temperature sensor providing a temperature signal for controlling the temperature of the fusing operation to a predetermined target temperature. A common problem encountered in heating the fuser relates to a temperature difference, as measured at different axial locations along the roller, known as axial temperature droop, which may result in gloss variations of the image on the media or other problems. The thermal mass of a heated roller for the fuser, i.e., the fuser roller, typically may be greater at the ends where the roller may be provided with supporting journals, bearings, bushings and drive gears, such that heat flow from the heated roller may be greater at the ends than at a central portion of the roller. In addition, convective and radiated heat energy losses may also occur at the ends of the roller, resulting in the temperature at the ends of the roller tending to decrease more than the central portion of the roller under some conditions.
One solution to axial temperature droop in prior art rollers has been to construct a roller with a relatively thick metal core, providing a relatively large thermal mass to reduce axial temperature droop. The higher thermal mass roller may require a longer warm-up time from room temperature to printing temperature, and the thicker core may cause excessive temperature overshoot after completion of a print job as heat provided from the lamp during the print job continues to pass from the center of the roller to the exterior surface of the roller.
U.S. Pat. No. 6,118,969 describes a fuser roller for eliminating or reducing fuser droop. The described fuser roller includes a distributed mass in which a hollow cylindrical roller is provided with a greater thermal mass per unit length at a center portion of the roller than the thermal mass per unit length of the end portions. A greater thermal mass in one portion of a roller may be accomplished by providing a higher thermal capacity material in the center portion than at the end portions, or by forming the center portion of the roller with a greater thickness than is provided at the end portions.
Providing a fuser roller core with a large thermal mass may result in an undesirable increase in the time for the fuser to warm up to an operating temperature. One prior art solution to providing efficient heating of the roller comprises providing a thin metal, typically steel or aluminum, fixing roller core and including a heater lamp having a boosted filament, which produces more heat at the ends than in the center of the lamp. However, one problem observed during certain conditions of operation of such a fuser roller is that the axial temperature droop may exceed a desired fuser temperature operating window. For example, the fuser roller may exhibit a large axial temperature droop during steady state operation in a standby or print mode of operation. It is typical to provide a temperature sensor for sensing the temperature adjacent one of the end portions of the roller as a feedback temperature for controlling power to the heating element for the fuser roller. When the end portion of the roller drops below the operating temperature, the heating element will be powered to deliver more energy in order to maintain the monitored end portion temperature at the operating temperature. Since the center portion of the roller exhibits less heat loss than the end portions, the temperature of the center portion may increase faster than the temperature at the end portions of the roller, thereby producing a large temperature differential along the axis of the roller.
Accordingly, there continues to be a need for a fuser in which axial temperature droop of a roller in the fuser may be minimized.